1. Field of the Invention
The present invention relates to a color tone variable film, a method for manufacturing the same, and a chromic element obtained by the manufacturing method. More specifically, the invention relates to a color tone variable film that contains various kinds of color tone variable compounds, such as an electrochromic coloring material or a photochromic coloring material, and changes color tone by energy application, a simple manufacturing method therefor, and a chromic element obtained using such a manufacturing method.
2. Description of the Related Art
As a use of chromic materials, shading materials, such as display elements, information recording materials, and concentration variable sunglasses are known. In recent years, however, the possibility of a new use, such as electronic paper, has been found as a use of electrochromic materials. Thus, a structure which is flexible in addition to quickly changing color tone at a low voltage has been required instead of a structure that is hard and slowly changes color tone, such as shading glass.
The application of such materials in which the hue variously changes by energy application has been attempted. However, for manufacturing a color tone variable film with known conventional compounds, a vapor phase method using a vacuum deposition facility has been commonly used. Thus, a technique by which the color tone variable film can be simply manufactured at low cost without using such a device has been desired. On the other hand, a color tone variable film has been formed by applying a chromic compound as a solution and drying the same. However, particularly in the case of polymer compounds, the solubility in a solvent is low, and the degree of freedom of molecular design is limited in many cases. Therefore, a technique by which a poorly soluble film can be finally formed even when compounds with high solubility are used has been desired.
From the viewpoint of portability, a drive voltage for changing the color tone is preferably lower. For example, in the case of electrochromic materials, driving at 3 V or lower by the electrodeposition of silver or bismuth has been reported (e.g., K. Shinozaki, SID'02 Digest, 39-41 (2002)). Furthermore, in an example in which a viologen derivative that is reduced to nanocrystalline titanium dioxide and develops color is adsorbed, driving at 1.2 V has been reported (e.g., A. Hagfeldt, N. Vlachopopulos, and M. Gratzel, J. Electrochem. Soc., 141, L82-L84 (1994)).
It is considered that the use of a tetracyanoquinodimethane (TCNQ) derivative that is more easily reduced than viologen or anthraquinone imide can achieve driving at a lower potential. An electrochromic material containing an anion radical of TCNQ is known. However, an electrochromic polymer having a TTF (tetrathiafluvalene) skeleton as the main component changes color by oxidation-reduction, and the anion radical of TCNQ which is a chemical species in which the TCNQ has been already reduced simply has had a function of assisting the color change of TTF. In contrast, an electrochromic element using the electrochromism of the TCNQ itself is disclosed, but vapor deposition is used for the film formation of TCNQ, whereby the manufacturing cost is high, and the electrochromic element is not suitable for flexible uses (e.g., Japanese Patent Application Laid-Open (JP-A) No. 60-200235). Moreover, the electrochemical redox behavior and the electrochromism of a film formed on a transparent electrode by spin coating a tetrahydrofuran (THF) solution of a polyester compound having a TCNQ skeleton have been reported (e.g., G. Inzelt, R. W. Day, J. F. Kinstle, J. Q. Chambers, J. Phys. Chem. 87, 4592-4598 (1983) and G. Inzelt, R. W. Day, J. F. Kinstle, J. Q. Chambers, J. Electroanal. Chem., 161, 147-161 (1984)). However, since the polyester having the TCNQ skeleton has low solubility in a solvent, a coating film containing active ingredients in a sufficient concentration cannot be formed by spin coat application and the function as an electrochromic material is insufficient.
As the color tone variable material, an electrochromism material in a near-infrared region is also useful from the viewpoint of heat insulation effects or recording of invisible information. An electrochromic coloring material of tungsten oxide has absorption in the near-infrared region but has the same problem as above in that the film formation method is limited to a vapor phase method. Thus, the application of the electrochromic coloring material of tungsten oxide to flexible materials has been difficult. In contrast, the electrochromism of a spin coat film of a polymer having a pendant anthraquinone imide skeleton has been reported in recent years (e.g., Y. Zheng, J. Zheng, L. Dou, W. Qiao, X. Wan, J. Mater. Chem. 19-8470 (2009)). However, since the compound is more difficult to reduce than TCNQ, there is a problem in that a drive voltage is higher than that of the electrochromic element using a TCNQ derivative.
Thus, in order to produce a flexible color tone variable film using an electrochromic coloring material, a uniform film containing a sufficient amount of the electrochromic coloring material even in the case of a thin film needs to be formed by an application method instead of a vapor phase method, and such a technique has been desired.
Thus, by obtaining a flexible color tone variable film, application to various fields, such as an electrochromic element, can be achieved by selecting the type or amount of a color tone variable compound to be blended in the color tone variable film.